If you don't already have a Google Voice account, making outbound phone calls through Hangouts doesn't automatically give you a full Google Voice account. The initial Google Voice account that's created when making phone calls from within a video call doesn't contain all of the features of Google Voice, such as a unique Google phone number. This initial account allows you to manage the basics for calling phone numbers in a video call, such as call credit. If you're a Gmail or Google Voice user, your call credit and call history will be shared across all three products. At the moment, users outside the US cannot upgrade to a full Google Voice account.